The possibilites that come to mind are:

1) Are any of the parts anodized aluminum alloy?

2) Someone monkeying around with metal polish (Flitz or similar) will frequently overpolish the easy to reach parts, causing this variation.

3) An improperly removed surface coat of rust can cause this. I have a 6 1/2" .22 cal. Ruger Single Six that is like this, but that Single Six is a working handgun and has been used so much over the past 25 years that I've worn the rifling pretty shallow and really should have had it rebarrelled several years ago. The reason that Single Six has such variability in the bluing is because of holster wear and on several occasions I've had a light surface coat of rust on it which I removed with #0000 steel wool. Otherwise the blueing has actually held up pretty well. The sides of the hammer which are not really polished or finished at the factory have also taken on a "blue" from a sweaty thumb over the years.

4) It can vary depending on how consistent the guy at the factory was when buffing the bare metal.

Richard